Nucleation within Steep Composition Gradients

     This Department of Energy (DOE) project titled "Experimental and Computational Studies of Crystal Nucleation in Composition Gradients" aims to build on our previous work and established capability using nanocalorimeters to study intermetallic reactions. This project is a continuing collaboration with Dr. David LaVan of the National Institute of Standards and Technology. Nanocalorimeters are small, microfabricated devices which enable rapid heating and precise thermal measurement of small samples.

     In previous efforts, extremely sharp gradients in crystalline Al/Ni multilayers provided a valuable system for studying nucleation conditions due to the precision control of the thermal history. We now apply the same processes to binary metallic glass-forming systems, allowing for a broader control of the gradient.

     The experimental component of the project studies the roles of concentration gradient on nucleation rates and temperatures using isochronal and isothermal nanocalorimetry. Isothermnal nanocalorimetry will be performed in situ in a TEM to measure nucleation rates. This will be coupled with an analytical modeling effort aimed at predicting nucleation. The theory developed in the project will be a notable improvement on our existing understanding of how nucleation occurs in sharp concentration gradients, with the experiments representing the first rigorous and systematic verification of such a model.

                                                       DOE1

Nanocalorimeter Chip: (a) Cross-sectional schematic and (b) top-down micrograph of a fabricated nanocalorimeter chip.

                                                       NSF2

     A simple example of a nanocalorimetry result, here showing the heat capacity spike associated with the melting of Al nanoparticles. Note the sensitivity of the nanocalorimeter with absolute heat capacity in micro-Joules per Kelvin.